Crystal contact of which one element is mainly silicon



April 29, 19 7 D. E. JONES ET AL I CRYSTAL CONTACT OF WHICH ONE ELEMENTIS MAINLY SILICON Filed Aug. 10, 1942 a v I w. U M h ,7 v 3 FIG. I.

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Patented Apr. 29, 1947 CRYSTAL CQNTACT OF WHICH ONE ELEMENT IS MAINLYSILICON Douglas Eniield Jones, Harrow, Charles Eric Ransley, Sudbury,.lohn Walter Ryde, London W. 14, and Stanley Vaughan Williams, Kenton,England, assignors to-The General Electric Company Limited, London,England Application August 10, 1942; Serial No. 454,290 In Great BritainAugust 20; 1941 11 Claims.

This invention relates to electrical crystal contacti devices of thetype comprising a semiconducting: crystal and a metal (usually a metalpoint) in contact with it. Such contacts can be used as rectifiers ofalternating current or as mixers, that is to'say as non-linearimpedances to which are applied an oscillation of frequency f1 andanother oscillation-f. frequency f2, so that oscillations are producedhaving frequencies (pfiiqfz) where p and q are integers.

Objects of the invention are to provide improved-processes for thepreparation of the'crystal in such contact devices, and crystal contactelements andv wave-signal translating devices embodying such crystalsthat are more uniform or more efi'icient or both than those of the saidtype ..know-n= hitherto. The uniformity aimed at means thattheelectrical characteristic of the contact should be as nearly aspossible independent of the position on the crystal of a metal point.forming the otherelement of the contact. Efiiciency generally requires ahigh ratio of impedance in one direction (reverse impedance) toimpedance in the other direction (forward impedance). It may alsorequire that the absolute values of the forwardimpedance should be highor, alternatively,that they should be low. It may also require that theforward direction should be that'in which positive charge flows from thecrystal to the metal (positive contact) or that it should be that inwhich negative charge flows from the crystal to the metal (negativecontact).

When-the-contact is used'as a mixer; efficiency requires-that thesignal-to-noise ratio should be relatively high. Efficiency alsorequires that the performance of the" contact judged by any oftheforegoing criteria should deteriorate as little as possible with useor with age.

Silicon is one of the semi-conductors most widely used in crystalcontacts. that the said objects of the invention can be attained byintroducing into substantially pure silicon a small proportion of.aluminium. or beryllium or bothof these metals which form asolidsolutionwith silicon and have extremely stable oxides. theratio of thenumber of atoms of the additive metal to 1 the number of atoms ofsilicon in the product: is not greater thanl/50. Impurities, suchas'iron, which are often present in commercial silicon are generallyundesirable and shoulcl'be' removed before the additive metal isintroduced.

Crystal contact elements-resulting from the herein disclosed" processare the'subl'ect matter We have found The term small impliesthatofdivisional application Serial No. 736,0'94;-filed- March 20';1947, and entitled Crystal contacts of which one elementis mainlysilicon.

If the starting material is the commercial silicon at present available,the introduction of: the-selected additive metal must be preceded b y'a;purification; This purification can be efiected by subjecting commercialsilicon to the-process-- described by N; P. Tucker in the Journal of theIron and Steel Institute, vol. CXV(1), p. 412-;- 1927. This processremoves only those impurities that are not contained in thesiliconcrystals; but occur as an additional-phase, usually in the intersticesbetween the' si1icon -crystals.-- Commercial silicon is usually'preparedby-cooling a melt slowly; thenpractically all the deleterious impuritiesare in the interstices-andcan be "re-'- moved by Tucker-s process. Ifthat-processfails with any sample of silicon, it isprobably because thesilicon has beenquenched rapidly; then' the silicon should be re-meltedand cooled slowly before it is I subjected to the Tucker process:

The additive metal is preferably introduced-"by melting 'amixture-ofthe:pure"silicon and the additive. During'this process caremust be taken not to introduce undesired impurities; Those that might bederived from the air "(asmal1 quantity of oxygen is not'necessarilydeleterious) can be avoided by melting in'a suitable atmosphere; themost suitable we have found is avacuum, that is to say residual gas at-apressure of not more" than 0.001 mm. of mercury. But it is difliculu orperhapsimpossible, to findany crucible to contain the melt that does notreact slightly with the melted silicon. This difii-- culty can beovercome by'using a cru'cib1e"con'- sisting of or lined With pureberylliaq' forthen' the only impurity introduced from" the" crucible isberyllium,- which is not harmful but beneficial. It might be possible"to introduce from the crucible all the additive'metal" characteristic ofthe invention; but in view of" patent" application Serial No.446;3l9,filed'June 8, 1942, We exclude from the scopeof this invention anyprocess-of. manufacturing the mainly-silicon element ofacrystal contactwhich comprises only the steps mentioned in the claims of thespecification ofthat" application. If therefore the first stepin the'processconsists in grinding relatively im pure silicon to a-fine powderand treating it with chemicalreagents soas to produce a relativelypureproduct, the process will not lie Withinthe scope of the presentinvention;unlesspartat leastofmthe additive metal is derived" from a'source other than a container in which th silicon is melted. But it isto be understood that the introduction of additive from the container isalways to be taken into account when the amount of additive to beintroduced from other sources is considered. Whether additive is or isnot introduced from the container, that otherwise introduced may bealuminium or beryllium or a mixture of the two.

In order that the resulting crystals may have the best qualities, it isgenerally necessary and always desirable that, after the introduction ofthe additive, the crystals should be subjected to an appropriatetreatment modifying their surface. One part of this process is acontrolled oxidation; this part can be omitted only if the necessaryoxidation has been effected during the melting or the cooling ofthemelt. We have occasionally succeeded in producing satisfactoryresults by this method; but it is difiicult to control, and we greatlyprefer to remove any surface oxide from the cooled melt withhydrofiuoric acid and then to heat th product in a definitely oxidisingatmosphere. Further We prefer not to attempt to produce the optimumsurface oxidation by the heating alone; it is usually better to oXidisethe surface somewhat more than is required and then to remove some (butnot all) the layer of oxide with hydrofluoric acid.

Other surface treatments are also permissible and often desirable. Thusit is usually preferable to polish the fragments of the melt, or atleast that part of them which is to contact with the metal point, beforethe controlled oxidation. Again, crystals are often soldered into metalcapsules; then it is desirable to plate with metal that part of thecrystal that is to lie in the capsule, in order that the solder mayadhere to it.

One method of performing the invention will now be described by way ofexample, with reference to the accompanying drawings, in which:

Figure 1 shows the completed crystal contact device;

Figure 2 shows a silicon fragment temporarily mounted in readiness for astep in the process of preparation; and

Figure 3 illustrates the step of mounting the same element in a capsule.

Referring to Figure 1, the completed crystal contact device consists ofa ceramic tube In capped at its ends respectively by a metal capsule l Iand a bush [2. The crystal body 13 is set in the capsule H by means of alow-melting alloy l4. One or more metallic contact elements [5 carriedby a metal rod 16 co-operate with a prepared face I! of the crystalbody, the rod l6 being held fast in the bush l2.

The starting material is commercial silicon which has been ground andpassed through a 200 mesh sieve; the material contained the followingmetals in the following amounts reckoned as oxides: (.Al2O3-I-ZIO2)1.01%, FezOs 0.96%, TiO'2 0.021%, MnO 0.08%, BaO 0.03%, CaO 1.13%, CuO0.002%, MgO less than 0.002. In step 1 this material was purified by theaforesaid method described by Tucker. The resulting material had thefollowing composition: (AlzOa+ZrOz) 0.03%, F820 less than 0.005%, T102less than 0.0005%, MnO less than 0.001%, BaO less than 0.003%, CaO0.008%, CuO 0.001%, MgO less than 0.002%, In step 2, the purifiedmaterial is melted in vacuo (pressure less than 0.001 mm. of mercury) ina pure beryllia crucible, together with aluminium in an atomicproportion of 1 Al to 400 Si. The

. 4 beryllia was fired to 1400 0., ground and'washed with nitric acidbefore being made into a crucible.

In a further step 3 a fragment of the cooled melt has a plane part ofits surface highly polished by any of the processes customary amongmetallurgists, for example grinding with emery of increasing finenessand finally polishing with a1umina or magnesia or both. In a furtherstep 4 the whole surface of the fragment is then dipped for 5 minutesinto commercially pure 40% hydrogen fluoride diluted with an equalquantity of water. The main purpose of this treatment with hydrofluoricacid is to prepare th polished surface for the subsequent oxidation;accordingly it is not necessary, though it is usually convenient, thatthe unpolished part of the surface should be clipped.

In a further step 5 the necessary surface oxidation is performed. Forthis purpose the body resulting from step 4 is placed on a flat silicatray (together possibly with other bodies at the same stage ofpreparation) and introduced into a silica tube furnace filled with airat a region thereof maintained at 1050 C. There is a definite optimumtemperature for this treatment, the temperature should be observed, e.g. by a th'ermocouple, and kept as near to 1050 C. as possible. The bodyis maintained in the said region for 2 hours and then allowed to cool.An atmosphere of oxygen at a controlled pressure may be substituted forthe air in the furnace; then the time of heating will depend on thepressure of the oxygen.

In a further step 6 the treatment given in step 4 is repeated. Now partof its object is to prepare the unpolished part for step '7; accordinglythe unpolished part must be dipped. In the process now being describedin detail the dipping of the polished part is equally necessary, inorder that part of the oxide layer formed in step 5 should be removed;but, as explained heretofore, it might possibly not be in a modificationof the process. After the clipping, the body is washed with water.

In a further step 7 a metallic layer is deposited on some or all of theunpolished surfaces. The layer must not be deposited On the polishedsurface; accordingly, as shown in Figure 2, this is first protected bycoating the flat end of a rod is with a layer of adhesive l9, which maybe the material known as monostyrene, and pressing the polished surfacell of the body l3 against this layer. The exposed unpolished surface isthen coated with copper by a method described by Bedel in ComptesRendus, vol. 192(1931) page 802. It consists merely in dipping thesurface for 5 to 10 seconds into a solution of cuprous oxide in 20%hydrofluoric acid; the copper layer deposited may be subsequentlythickened by electrolysis. molybdenum by this process; accordingly therod [8 may be conveniently of tungsten or molybdenum. Ihe coated surfaceis then washed and dried. The body can then be mounted in the metalcapsule ll by means of solder or some other suitable low-melting alloyintervening between the metal layer and the capsule. For this purposethe rod I8 is inserted in the tube l0 so that the crystal enters themolten alloy M in the capsule H, which is temporarily placed on an endof the tube. After the alloy has set, the rod [8 is detached from thebody and the polished surface of the body cleaned from the adhesive bywashing with a suitable solvent, such as benzene or ethyl acetate. Themounted crystal body is then ready Copper is not deposited on tungstenor to lthe :axis. .of the wire.

for final :assembly into :the structure-shown in "Figured.

:usually be obtained, but then the forward'im- L pe'dance isgreater. Thecontact will usually stand :without appreciable deterioration thepassage zacross ityforl a period of to '60 secs, of acurrentlimitingcurrentyof 40 to 80-ma. Comparable average figuresi for the commercialsilicon that was thesstarting, material, -polished as described"hereinbefore 1 but not treated otherwise, :are forward impedance l80ohms, ratio of reverse toforward impedance 5, limiting current 5- ma.

If alowertforward impedance is required, the 5 pressure between crystal.and wire 'maybe increased to -50 gm. weight. A forward impedance in theneighbour-hood' of 100 ohms can then be obtained, but the averageratioof reverse to forward impedanceis likely to be 10-20, and the limitingcurrent 30-60 ma.

If a high limiting current is not so important as th impedancecharacteristic, the aforesaid sheared tungsten point may be replaced bya fine tungsten point obtained by the well known process of etching withLangmuirs solution.

One advantage of crystals prepared according to the invention is thatthey are so uniform that it is often possible to locate two metal points(whiskers) in parallel on the same crystal without adjusting themseparately, as shown in Figure 1, and yet to obtain in this way animproved characteristic. The improvement is usually not so much in theimpedance as in the limiting current that the contact will stand withoutdeterioration. No novelty is claimed for the bare suggestion thatwhiskers on the same crystal may be connected in parallel; but, so faras we are aware, no disclosure has been made before how crystals mightbe prepared that are at once of high effipractically useful.

We claim:

1. In the manufacture of an electrical crystal contact device of thekind in which the semiconducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the step of introducing into silicon that issubstantially free from the metallic impurities usual in commercialsilicon a determinate small quantity of at least one of the two metalsaluminium and beryllium, which additive metal forms a solid solutionwith the silicon.

2. In the manufacture of an electrical crystalcontact device of the kindin which the semiconducting crystal contact element is mainly silicon,the production of said silicon contact element by a process whichincludes the step of introducing into silicon that is substantially freefrom the metallic impurities usual in commercial silicon, a quantity ofat least one of the two metals aluminium and beryllium, said quantitybeing such that the ratio of the number of atoms of said additive metalto the number of atoms of-siliconsinithe pro duct isnot greater than 11/50.

*"3. In themanufacturemf an electrical crystal contact devicegof thekind i in which lthe semiconducting crystal contact element is mainlysi1icon,,theproduction of saidsilicon'contact element by a process whichincludes the-steps vof adding to solid silicon that is substantiallyfree from the metallic impurities usual in commercial silicon'adeterminate small quantity-of atleast one of the two metalsaluminium andberyllium also in solid form, and thereafter meltingthemixture. b

4. In themanufacture of an electrical crystal zcontactndevice of thekind in :which the semiconducting crystal contact element is mainly,silicon, the'production of said silicon contact element by a-processwhich includes the steps of adding to solid silicon that issubstantially free from the metallic impurities usualin commercialsilicon a 1 determinate small quantity of J at least .one of the 'twometalsaluminium and beryllium also insolid form, and thereafter:meltingthemixture ina container at least the interior surfacesof thewalls of which are composed of pure'beryllia.

5. In the manufacture of an electrical crystal contactdevice of the kindin -whichthe semi- --conducting crystal contact element is mainlysilicon, the production of saidsiliconcontact element'bya processiwhi'chincludes the steps of introducing into: silicon vthat. is substantiallyfree from the metallic impurities usual in commercial silicon adeterminate small quantity of at least one of the two metals aluminiumand beryllium, which additive metal forms a solid solution with thesilicon, and subjecting to a controlled process of surface oxidation thebody of silicon into which ciency and also so uniform that thesuggestion is said additive metal has been introduced.

6. In the manufacture of an electrical crystal contact device of thekind in which the semi-conducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the steps of introducing into silicon that issubstantially free from the metallic impurities usual in commercialsilicon a determinate small quantity of at least one of the two metalsaluminium and beryllium, which additive metal forms a solid solutionwith the silicon, treating with hydrofluoric acid a surface of the bodyof silicon into which said additive metal has been introduced,thereafter heating said body in an oxidising atmosphere, and treatingsaid surface again with hydrofluoric acid so as to remove some but notall of the oxide layer formed during said heating,

7. In the manufacture of an electrical crystal contact device of thekind in which the semiconducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the steps of introducing into silicon that issubstantially free from the metallic impurities usual in commercialsilicon a determinate small quantity of at least one of the two metalsaluminium and beryllium, which additive metal forms a solid solutionwith the silicon, polishing part at least of the surface of the body ofsilicon into which said additive metal has been introduced, andsubjecting to a controlled process of surface oxidation said polishedpart.

8. In the manufacture of an electrical crystal contact device of thekind in which the semiconducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the steps of introducing into silicon that issubstantially free from the metallic impurities usual in commercialsilicon a determinate small quantity of at least one of the two metalsaluminium and beryllium, which additive metal forms a solid solutionwith the silicon, and coating with a metallic layer part of the surfaceof the body of silicon into which said additive metal has beenintroduced.

9. In the manufacture of an electrical crystal contact device of thekind in which the semiconducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the steps of introducing into silicon that issubstantially free from the metallic impurities usual in commercialsilicon a determinate small quantity of at least one of the two metalsaluminium and beryllium, which additive metal forms a solid solutionwith the silicon, polishing part of the surface of the body of siliconinto which said additive metal has been introduced, affixing the flatend of a rod by means of an adhesive to said polished surface part,dipping said body into a solution of cuprous oxide in hydrofluoric acid,removing said rod from said body, and cleaning said surface part freefrom adhesive.

10. In the manufacture of an electrical crystal contact device of thekind in which the semi-conducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the steps of introducing into silicon that issubstantially free from the metallic impurities usual in commercialsilicon a determinate small quantity of at least one of the two metalsaluminium and beryllium, which additive metal forms a solid solutionwith the silicon, coating with a metallic layer part of the surface ofthe body of silicon into which said additive metal has been introduced,and mounting said body in a metal holder to which said metallic layer issoldered. I

11. In the manufacture of an electrical crystal contact device of thekind in which the semi-con;- ducting crystal contact element is mainlysilicon, the production of said silicon contact element by a processwhich includes the steps of introducing into silicon that issubstantially free from the metallic impurities usual in commercialsilicon a quantity of additive metal such that the ratio of the numberof atoms of said additive metal to the number of atoms of silicon in theproduct is not greater than 1/50, polishing a part of the surface of thebody of silicon so produced, treating said polished part withhydrofluoric acid, thereafter heating said body in an oxidisingatmosphere, and

treating said'surface again with hydrofluoric acid so as to remove onlysome of the oxide layer formed by said heating.

DOUGLAS ENFIELD JONES. CHARLES ERIC RANSLEY.

JOHN WALTER RYDE.

STANLEY VAUGHAN WILLIAMS.

